Le train retardé arrive tard.

In French, most descriptive adjectives come after the noun.

So:

• le train retardé = the delayed train

That word order is normal. French does have some adjectives that often come before the noun, but retardé is not one of them.

Here, retardé means delayed.

It is related historically, but you should be careful: when talking about people, retardé can sound offensive or outdated, much like English retarded.

For things like trains, flights, deliveries, or projects, it can simply mean delayed:

• un vol retardé = a delayed flight
• un train retardé = a delayed train

So in this sentence, it is perfectly understandable.

Because retardé is being used as an adjective modifying train, not as the main verb of the sentence.

Compare:

• Le train retardé arrive tard. = The delayed train arrives late.
• Le train est retardé. = The train is delayed.

In the sentence you were given, the main verb is arrive.
So the structure is:

• Le train retardé = the delayed train
• arrive tard = arrives late

Because after a subject, French normally needs a conjugated verb, not the infinitive.

• arriver = to arrive (infinitive)
• arrive = arrives / is arriving (3rd person singular present)

Since the subject is Le train retardé, the verb must match it:

• Le train retardé arrive tard.

If you used arriver, it would be ungrammatical in this sentence.

They do different jobs in the sentence.

• retardé = an adjective meaning delayed
• tard = an adverb meaning late

So:

• le train retardé describes what kind of train it is
• arrive tard describes how/when it arrives

A simple way to see it:

• retardé modifies the noun
• tard modifies the verb

Both are connected to lateness, but they are not used in exactly the same way.

• retardé = delayed
• en retard = late / behind schedule

Examples:

• Le train est retardé. = The train is delayed.
• Le train est en retard. = The train is late.

For trains, many native speakers would very often use en retard or a du retard in everyday speech:

• Le train est en retard.
• Le train a du retard.

So retardé is understandable, but en retard is often more common in real-life train situations.

It is grammatically correct and understandable, but it may sound a little less natural than some other ways of saying it.

A native speaker might more often say:

• Le train est en retard.
• Le train a du retard.
• Le train arrive en retard.

Your sentence is still useful because it clearly shows two different ideas:

• the train is delayed
• it arrives late

So it works well as a learning example, even if everyday French may prefer other phrasing.

Yes. As an adjective, retardé agrees in gender and number with the noun it describes.

Forms:

• masculine singular: retardé
• feminine singular: retardée
• masculine plural: retardés
• feminine plural: retardées

Examples:

• Le train retardé
• La rame retardée
• Les trains retardés
• Les lignes retardées

A careful pronunciation is roughly:

lə tʁɛ̃ ʁə.taʁ.de a.ʁiv taʁ

A few useful notes:

• train has a nasal vowel, something like tran but nasal
• retardé ends with a clear -dé
• arrive sounds like a-riv
• tard has a silent final d

So tard is pronounced roughly like tar.

Le is the masculine singular definite article, meaning the.

It tells you we are talking about:

• a specific train
• or a train already known from context

So:

• Le train = the train
• Un train = a train

If you changed it to Un train retardé arrive tard, the meaning would become more general: A delayed train arrives late.